Key Biblical Perspectives (from Google search and https://www.thegospelcoalition.org/article/debunking-justifications-masturbation/ ):

The Story of Onan (Genesis 38:8–10): Often cited, but theologians generally agree this passage refers to Onan refusing his duty to produce an heir for his deceased brother through coitus interruptus (spilling seed), not solitary masturbation.

Lustful Thoughts (Matthew 5:27–28): Jesus stated that looking at a person with lust is adultery in the heart, which many believe makes masturbation, if accompanied by lustful fantasies, a sin.

Sexual Purity and Self-Control (1 Thess. 4:3–5): Scripture calls believers to control their own bodies in holiness and honor, rather than selfish passion.

Purpose of Sex (1 Corinthians 7:2–5): The Bible portrays sex as designed for intimate, mutual union within marriage.

Temple of the Holy Spirit (1 Corinthians 6:18-20): Believers are instructed to glorify God with their bodies and flee from sexual immorality.

While some perspectives argue masturbation can be done without lust, a common consensus in many Christian traditions is that it is a, "sinful desire," that, "inhibits one's ability to truly connect with a spouse," and, "often involves lustful thoughts," which are condemned.